The epithelial sodium channel (ENaC) has been proposed to be a vital regulatory mechanism in renal sodium transport and arterial pressure control and has clear disease implications for humans. Liddle~s syndrome, a condition characterized by excessive sodium reabsorption, volume overload, and hypertension is caused by activating mutations in EnaC subunits (1-4). In contrast, loss-of- function ENaC mutations have been defined in families with pseudohypoaldostreronism type I (PHA1) associated with salt wasting, dehydration, and metabolic acidosis (5,6). Direct assessment of ENaC function in vivo has not therefore been possible. The overall goal of this proposal is to directly quantitate the relative importance of ENaC in renal sodium transport and arterial pressure control. To achieve this goal, we proposed the following Specific Aims. Aim # 1 to generate and characterize mice with an inactivated beta-subunit of EnaC. Aim # 2 to generate and characterize transgenic mice with overexpression of a mutated GammahEnaC. Aim # 3 to characterize the role of ENaC in the acute and chronic regulation of renal sodium handling and arterial pressure in genetically manipulated mouse models. We will utilize the powerful methods of gene targeting and transgenic technology to generate murine models with under-and overexpression of ENaC activity. Generation of transgenic mice overexpressing a mutated Gamma hENaC has already been achieved, and correctly targeted embryonic stem (ES) cell clones with an inactivated Beta EnaC allele have been pro9duced. This set of constructs will uniquely enable us to dissect the relative importance of ENaC. In the proposed studies we will utilize a comprehensive approach which will entail molecular biological, electrophysiological, and conscious whole animal physiology experiments. Recent advances in our ability to culture murine inner medullary collecting duct (IMCD) cells will allow us to conduct direct electrophysiological studies of sodium transport in the distal nephron. Finally, we have developed the expertise and tools necessary to measure cardiovascular and renal function parameters in conscious, chronically instrumented mice, and thus have the ability to perform essential physiological experiments. Information gained from these studies will provide important new insights into the mechanisms involved in the physiology of sodium homeostasis and arterial pressure control.